Temperature-induced deactivation of catalysts containing platinum group metals (PGMs) such as platinum, palladium, and rhodium is a great challenge facing environmental catalysis today. Historically, in a standard preparation, PGM particles have been deposited on thermally durable metal oxide supports (e.g., alumina (AI2O3), zirconia (ZrO2), ceria (CeO2), ceria-zirconia composites (CeZrOx) and the like) by impregnating the supports with solutions containing ions of PGMs, wherein the ions were provided by precursors, such as salts of the desired PGM. Impregnation methods typically lead to the formation of PGM particles with an average diameter of less than or equal to about 5 nanometers (nm) on the metal oxide support. Upon hydrothermal aging, these small particles experience fast deactivation. One mechanism of deactivation is the agglomerating of these small PGM particles, leading to agglomerated group of particles with diameters, e.g., of several hundreds of nanometers. Another mechanism of deactivation can be a solid state reaction between small PGM particles and the metal oxide support, forming inactive PGM-support mixed oxides (for example, RhAlO3). Both of these deactivation mechanisms are associated with small PGM particle size, for example, less than or equal to about 5 nm in the initial fresh state.
International Patent Application Publication No. WO 2011/017139 is directed to “preparation of diesel oxidation catalysts via deposition of colloidal nanoparticles,” and exemplifies the formation of platinum nanoparticles on a microparticle alumina support material using chemical fixation by pH adjustment. By use of a microparticle alumina support material, the platinum nanoparticles are primarily located on the surface of the support material (largely outside of the pores of the support material). U.S. Patent Appln. Publ. No. 2012/0263633 describes metal oxide support materials containing nanoscaled iron-platinum group metal particles having a particle size from 0.5 nm to 10 nm, wherein at least 70% of the nanoscaled iron-platinum group metal particles are located on an outside surface layer of the metal oxide support material. The nanoscaled particles of this 2012/0263633 reference originated from precursor soluble salts.
There is a continuing need in the art to provide catalytic articles that provide excellent catalytic activity, thermal stability, and/or efficient use of components such as PGM particles.